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MAC seminar
 

MAC seminar

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seminar on introduction to MAC protocols

seminar on introduction to MAC protocols

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    MAC seminar MAC seminar Presentation Transcript

    • Comparison of MAC protocols in wired systems and wireless systems Mridula Sharma 19.07.2010 Comparison of MAC protocols in wired systems and wireless systems
    • Outline
      • Introduction
      • Multiple Access Protocols
      • Wired Network
      • Wireless Networks
      • Conclusion
      • References
      Comparison of MAC protocols in wired systems and wireless systems
    • Introduction
      • Large class of networks is built on top of broadcast channels
      • Problem : if you’re sharing a channel, then two stations may decide to start frame transmission at the same time
        • Frame collision , which means rubbish on the wire
      • Solution : Allocate the channel to one of the competing stations
      • Allocation can be of two types:
        • Static: FDM and TDM
        • Dynamic
      Comparison of MAC protocols in wired systems and wireless systems
        • Protocol to determine who goes next on a multi-access channel belongs to the
        • Medium Access Control (MAC) sublayer
          • Especially important for LANs
          • Provides addressing and channel access control mechanisms
    • Introduction Comparison of MAC protocols in wired systems and wireless systems
    • Introduction Functions of MAC layer Comparison of MAC protocols in wired systems and wireless systems
      • On transmission, assemble data into a frame with address and error detection fields
      • On reception, disassemble frame, and perform address recognition and error detection
      • Govern access to a LAN transmission medium
    • Multiple Access Protocols
      • ALOHA
      • Carrier Sense Multiple Access (CSMA) Protocols
      • Collision Free Protocols
      • Limited Contention Protocols
      • Wavelength Division Multiple Access Protocols
      • Wireless LAN Protocols
      Comparison of MAC protocols in wired systems and wireless systems
    • ALOHA
      • Radio-based communication network
        • Developed in 1970s at the University of Hawaii
        • Applicable to any system with competing uncoordinated users
      • Two versions
        • Pure ALOHA
        • Slotted ALOHA
      Comparison of MAC protocols in wired systems and wireless systems
    • Pure ALOHA
      • Let users transmit whenever they have data to be sent !!!
      • If a collision occurs, finish your current transmission and retry later
      • Performance is maximized if all frames have the same size
      • Random waiting time required else collisions occur
      Comparison of MAC protocols in wired systems and wireless systems Fig. : Frames transmitted at completely random intervals
    • Pure ALOHA- Efficiency
      • Probability that k frames are generated during a given frame time with G attempts per packet time:
      • Let P0 be the probability that frame does not suffer from collision
        • Throughput , S= G.P0
      • V ulnerable period = 2Tframe
      • Probability that a frame will not be damaged during two frame times
      • long is:
      Comparison of MAC protocols in wired systems and wireless systems
    • Slotted ALOHA Comparison of MAC protocols in wired systems and wireless systems
      • Fr ame transmission can only start at fixed times
      • Since the vulnerable period is now halved,
      Fig. : Throughput versus offered traffic for ALOHA systems
    • Carrier Sense Multiple Access (CSMA) Comparison of MAC protocols in wired systems and wireless systems
      • CSMA protocols do better than ALOHA : you monitor the channel
      • before and/or during transmission
      • 1-persistent
      • Nonpersistent
      • p-Persistent
      • .
    • Comparison of MAC protocols in wired systems and wireless systems Fig. : Comparison of channel utilization versus load for various random access protocols
    • CSMA with Collision Detection (CSMA/CD) Comparison of MAC protocols in wired systems and wireless systems
      • Sense the channel, but immediately stop transmission when you detect a collision
      • Saves Channel Bandwidth and time
      • Ethernet works like this!!!
      Fig. : CSMA/CD can be in one of the three states: Contention, Transmission or Idle .
    • Collision Free Protocols Comparison of MAC protocols in wired systems and wireless systems
      • Bit-Map protocol
      • Binary Countdown Protocol
    • Limited Contention Protocols Comparison of MAC protocols in wired systems and wireless systems
      • Contention systems are good when there’s not much going on
      • Collision-free systems are good when there’s generally a lot of traffic
      • What we really want is the contention strategy during light loads , and
      • collision-free strategy during rush hours !!!
      • Solution: Limited Contention Protocols
      • Dynamically regulate the number of competing stations during a contention period.
      • If there’s not much traffic, the first station will be immediately
      • allowed to transmit a frame.
      • With a lot of traffic, the strategy reduces to the bit-map protocol.
    • Limited-Contention Protocols The Adaptive Tree Walk Protocol Comparison of MAC protocols in wired systems and wireless systems
    • Wavelength Division Multiple Access Protocols Comparison of MAC protocols in wired systems and wireless systems
      • If you have a lot of bandwidth, just divide the channel into sub-channels, and dynamically allocate the sub-channels !!!
          • used in fiber optics
      Fig. : Wavelength Division Multiple Access
    • Ethernet Comparison of MAC protocols in wired systems and wireless systems
      • CSMA/CD based
        • sense the channel, wait until idle, and backoff if collision
      • Exponential Backoff
      • Near implementation of IEEE 802.3 protocol
    • 802.3 Frame Layout Comparison of MAC protocols in wired systems and wireless systems
      • Preamble : Seven times 10101010 is used to synchronize the receiver’s clock with that of the sender
      • Start : Just a delimiter to tell that the real info is now coming
      • Address : Generally 48-bit fields. Leftmost bit indicates ordinary or group addresses (multicast / broadcast). Second bit indicates global or local address
      • Length : Ranges from 0-1500. Frames should always be at least
      • 64 bytes
      • Pad : used to fill out the frame to the minimum size
      • Checksum : Calculated over the data field. CRC-based
    • Wireless Networks Comparison of MAC protocols in wired systems and wireless systems
      • Often, there are a number of base stations (a.k.a. access points) connected through guided media
      • Nodes can also group together to form an ad hoc network
    • Wireless LAN Requirements Comparison of MAC protocols in wired systems and wireless systems
      • Throughput
      • Number of Nodes
      • Connection to backbone LAN
      • Service Area
      • Battery Power Consumption
      • Transmission Robustness and Security
      • Collocated Network Operation
      • License-free Operation
      • Handoff/ roaming
      • Dynamic Configuration
    • Problems Comparison of MAC protocols in wired systems and wireless systems
      • There can be subtle interference:
      • Issue (a)
      • How can C be prevented from trying to transmit something to B ?
          • in that case it will ruin any receipt by B
          • hidden station problem
      • Issue (b)
      • How can we tell C that it is allowed to transmit to D, because this will not interfere with the communication from B to A ?
          • exposed station problem
    • Multiple Access with Collision Avoidance (MACA) Comparison of MAC protocols in wired systems and wireless systems
      • A first sends a Request To Send (RTS)
      • B answers with a Clear To Send (CTS)
      • C hears only RTS and can freely transmit, knowing it will not interfere with A’s transmission- solves exposed station problem
      • D hears only the CTS and keeps still for otherwise it would
      • interfere with B’s reception- solves hidden station problem
    • MACA for Wireless (MACAW) Comparison of MAC protocols in wired systems and wireless systems
      • G ets rid of Ethernet like unfairness associated with binary exponential backoff algorithms !!!
      • Acknowledges the importance of link layer acknowledgements
        • Makes the protocol from RTS- CTS-Data to RTS-CTS-Data-ACK
      • Significantly complex protocol
        • Performance loss with lightly loaded channel
        • Better throughput and fairer allocation in presence of high loads
    • Wireless LANs MAC Sublayer Comparison of MAC protocols in wired systems and wireless systems
      • Problem : How do we solve the hidden / exposed station problem ?
      • One way or the other, stations should not be allowed to continuously interfere with each other’s transmissions
    • Wireless LANs MAC Sublayer Comparison of MAC protocols in wired systems and wireless systems
      • IEEE 802.11 provides two methods to deal with this problem:
          • Distributed coordination : let the stations figure it out by using a
      • collision avoidance protocol (CSMA/CA)
          • Point coordination : there’s a central base station that controls who
      • goes first- mostly used
      • Collision detection is hard to implement on the wireless medium
          • radios cannot transmit and receive on the same frequency (half-duplex channels)
      • Solution : Use collision avoidance protocol
        • Two method like operations
          • Ethernet-like
          • MACAW
    • CSMA/CA Ethernet-like Comparison of MAC protocols in wired systems and wireless systems
      • Similar to non-persistent ALOHA
          • there random time was used before re-sensing the channel when busy
          • exponential backoff is used
      • Nowadays most MAC are reprogrammable by users
          • vulnerable to cheating !!!
    • Comparison of MAC protocols in wired systems and wireless systems Fig. : IEEE 802.11 Medium Access Control Logic
    • CSMA/CA- MACAW Comparison of MAC protocols in wired systems and wireless systems
      • MACAW : send RTS/CTS packets to see whether you should defer
      • transmission to avoid interference with another transmission
      • Evolution of MACA
      • Network Allocation Vector (NAV): it’s a virtual channel that a station
      • assigns to itself telling it to shut up!!!
      • Note: hidden station problem is solved but NOT exposed station problem!
    • CSMA/CA- point Coordination Comparison of MAC protocols in wired systems and wireless systems
      • In PCF the base-station polls the other stations, asking them if they have anything to send
      • It sends a beacon frame once every 10 or 100 ms
      • Sleep state to save battery
      • When base station transmits, there can be no hidden terminals
      • PCF and DCF can co-exist together
        • By carefully defining the inter-frame time interval
        • First the base station can poll the other stations
        • I f nobody replies, any station can acquire the channel
    • Comparison of MAC protocols in wired systems and wireless systems Fig. : IEEE 802.11 Protocol Architecture
    • 802.11 MAC Frame Layout Comparison of MAC protocols in wired systems and wireless systems
      • Type : Data, control, or management frame
      • Subtype : Are we dealing with RTS, CTS, an ACK, etc
      • DS : Is the frame entering/leaving the current cell?
      • MF : Frames are allowed to be fragmented to increase reliability. This bit tells whether more fragments are on their way
      • Retry : Is this a retransmission?
      • O : Stick to ordered delivery of frames
    • 802.11 MAC Frame Layout Comparison of MAC protocols in wired systems and wireless systems
      • Duration : Tells how long the transmission of this frame will take, allowing other stations to set their NAV accordingly
      • Addresses : Source/destination in a cell; and those of base stations outside the cell when dealing with inter-cell traffic
      • Sequence : Sequence number of this frame. 4 bits are used to
      • identify a fragment of a frame
      Note : 802.11 supports encryption at MAC level, Ethernet does not !!!
    • Broadband Wireless Comparison of MAC protocols in wired systems and wireless systems Goal : Use wireless connection between buildings i.e. avoiding the use of the local loop
      • More bandwidth is needed: 10-to-66 GHz frequency range
      • Full-duplex communication possible
      • M uch of the mobility stuff from 802.11 is not needed
      • Can be focused in directional beams ( IEEE 802.11 is omnidirectional )
      WiMAX (World Interoperability for Microwave Access) is a family of IEEE 802.16 aiming at replacing ADSL and cable modems
    • Broadband Wireless Comparison of MAC protocols in wired systems and wireless systems
      • MAC : the base station controls the systems by scheduling downstream and upstream channels
          • it is connection-oriented ( QoS needed by phone-companies)
      • Time Division Duplexing (TDD)
          • Downstream traffic is mapped onto time slots by the base station
          • Upstream traffic is more complex
            • The base-station pre-allocates it ( constant bit rate service )
            • The base-station periodically polls stations ( variable bit rate service )
            • No polling but subscribers have to contend ( best-effort service )
      Fig. : TDD frame contatinig time slots
    • Broadband Wireless- 802.16 Frame Structure Comparison of MAC protocols in wired systems and wireless systems Fig. : (a) a generic frame (b) a bandwidth request frame
      • Connection-oriented Service
      • Checksumming optional
      • Encryption is critical for the system
        • Managed at MAC level
    • Wireless Sensor Networks Zigbee Comparison of MAC protocols in wired systems and wireless systems
      • IEEE standard 802.15.4
      • N ame of a specification for a suite of high level communication protocols using small, low-power digital radios
      • MAC layer: CSMA/CA
    • Conclusion Comparison of MAC protocols in wired systems and wireless systems
    • Comparison of MAC protocols in wired systems and wireless systems Fig: Channel allocation methods and systems for a common channel
    • Conclusion Comparison of MAC protocols in wired systems and wireless systems
      • Simplest allocation schemes: FDM and TDM
        • Poor choices for large, variable or bursty traffic
        • Alternative: ALOHA (Dynamic Allocation)
      • Carrier Sensing in LANs and MANs led to variety of protocols
        • Binaray Countdown
          • Eliminates contention completely
        • Tree Walk
          • Reduces contention by dividing stations dynamically
      • Ethernet
        • Dominant in Wired LANs
        • Uses CSMA/CD
    • Conclusion Comparison of MAC protocols in wired systems and wireless systems
      • Wireless LANs
        • CSMA does not work here
        • MACA and MACAW
        • FHSS and DSSS
        • IEEE 802.11 suggests CSMA/CA
      • Broadband Wireless
        • 802.16, TDD
      • Wireless Sensor Networks
        • Zigbee
          • CSMA/CA
    • References
      • Andrew S. Tanenbaum, Computer Networks, Fourth Edition
      • William Stallings, Data and Computer Communications, Eighth Edition
      • Holger Karl, Andreas Willig, Protocols and Architectures for Wireless Sensor Networks
      • Paolo Costa, Compter Networks- Medium Access Control Sublayer
      • Alaa Muqattash, Marwan Krunz, CDMA-Based MAC Protocol for Wireless Ad Hoc Networks
      • Sushant Jain, Ratul Mahajan, Wireless LAN MAC protocols
      Comparison of MAC protocols in wired systems and wireless systems
    • Thank You!!! Any Questions??? Comparison of MAC protocols in wired systems and wireless systems